Digital electronic still cameras are rapidly becoming popular. With this kind of digital camera, by using a personal computer, one can easily perform processing of photographed image data. Due to advances in image sensors and market demand for more compact and inexpensive cameras, the need for miniaturization and price reduction of lenses for such cameras has become especially urgent.
Because CCD detector arrays (i.e., image sensor chips) used in electronic still cameras generally have properties which are highly sensitive to wavelengths on the infrared-side of the visible spectrum, there is a tendency for images of objects to be blurred. Therefore, a conventional construction has been adopted wherein an element such as an infrared-blocking filter is inserted in the imaging system in order to obtain clear images. For example, Japanese Laid Open Patent Publication H11-84232 describes a wide-angle lens having a construction wherein an infrared-blocking filter is placed between the photographic lens system and a CCD detector array in order to obtain clear images.
However, an infrared-blocking filter has become a rather expensive component in digital cameras where rapid price reductions have been occurring. Also, the need to secure sufficient space for placing an infrared-blocking filter results in decreasing the compactness of a camera lens which employs a separate element to serve as an infrared-blocking filter. Therefore, in order to design an imaging system which is low in cost and more compact, a construction as described in Japanese Laid Open Patent Publication H9-289644 has been proposed wherein infrared light is blocked by using an infrared-blocking coating applied to a lens surface rather than providing a separate infrared-blocking filter.
However, when using such an infrared-blocking coating, a problem tends to arise in that color reproducibility degrades with increasing field angles, due to the larger angles of incidence of the light onto the image sensor. The electronic still camera described in Japanese Laid Open Patent Publication H9-289644 is constructed such that a correction for color reproducibility can be performed using signal processing software. However, from the viewpoint of miniaturization and cost reduction, instead of this kind of complicated construction, lenses of simpler construction and better color reproducibility are desired.
Also, although it is not regarded as a problem in the conventional example where an infrared-blocking filter is used, there are cases where a ghost phenomenon occurs due to light being reflected by an infrared-blocking coating which then reaches the image surface. This phenomenon occurs more easily when an infrared-blocking coating is used rather than when a separate filter element is used. Thus, in a lens for an electronic still camera that uses an infrared-blocking coating, suppression of ghost images is strongly desired.